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Natural gas hydrates with locally different cage
occupancies and hydration numbers in Lake Baikal
M. Kida, Akihiro Hachikubo, Hirotoshi Sakagami, Hirotsugu Minami, Alexey
Krylov, S. Yamashita, N. Takahashi, H. Shoji, Oleg Khlystov, Jeffrey Poort,
et al.
To cite this version:
M. Kida, Akihiro Hachikubo, Hirotoshi Sakagami, Hirotsugu Minami, Alexey Krylov, et al.. Natural gas hydrates with locally different cage occupancies and hydration numbers in Lake Baikal. Geo-chemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2009, 10 (Q05003), pp.8 P. �10.1029/2009GC002473�. �hal-00688072�
Natural gas hydrates with locally different cage occupancies
and hydration numbers in Lake Baikal
Masato Kida
Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan
Now at Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology, 2-17 Tukisamu Higashi Toyohira-ku, Sapporo 062-8517, Japan. (m.kida@aist.go.jp)
Akihiro Hachikubo, Hirotoshi Sakagami, Hirotsugu Minami, Alexey Krylov, Satoshi Yamashita, Nobuo Takahashi, and Hitoshi Shoji
Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan
Oleg Khlystov
Limnological Institute, Siberian Branch, Russian Academy of Sciences, 3, Ulan-Batorskaya Street, P.O. Box 4199, Irkutsk 664033, Russia
Jeffrey Poort
Renard Centre of Marine Geology, Ghent University, Krijgslaan 281-S8, Ghent B-9000, Belgium
Hideo Narita
Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology, 2-17 Tukisamu Higashi Toyohira-ku, Sapporo 062-8517, Japan
[1] Knowledge of cage occupancies and hydration numbers (n) of naturally occurring gas hydrate in a local
environment is important for the improvement in global estimates of hydrate-bound natural gas. We report on local differences in cage occupancies and hydration number of gas hydrates from Lake Baikal. Natural gas hydrates of both structures I and II (sI and sII) and ranging in composition from pure CH4to mixed gas
hydrate containing up to 15% C2H6are compared. The average hydration numbers are n = 6.1 for the sI
CH4 hydrates recovered from the Malenky and Bolshoy mud volcanoes, n = 6.2 for the sI hydrates,
containing 3 –4% C2H6 recovered from the K-2 mud volcano, and n = 6.9 for the sII hydrate containing
about 15% C2H6recovered from the K-2 mud volcano. The differences in hydration number are due to the
differences in the small cage occupancy of CH4 among the samples studied.
Components: 4848 words, 4 figures, 2 tables.
Keywords: cage occupancy; hydration number; methane; ethane; natural gas hydrate; Lake Baikal.
Index Terms: 3004 Marine Geology and Geophysics: Gas and hydrate systems; 0714 Cryosphere: Clathrate; 4820 Oceanography: Biological and Chemical: Gases.
Received 3 March 2009; Revised 23 March 2009; Accepted 27 March 2009; Published 8 May 2009.
Kida, M., and et al. (2009), Natural gas hydrates with locally different cage occupancies and hydration numbers in Lake Baikal, Geochem. Geophys. Geosyst., 10, Q05003, doi:10.1029/2009GC002473.
Published by AGU and the Geochemical Society AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
1. Introduction
[2] Natural gas hydrates (NGHs), which are present
in marine and lacustrine sediments at water depths greater than about 500 m and permafrost layers, act as a large natural gas reservoir because of their widespread occurrence. NGH is an ice-like crystal-line clathrate compound that contains natural gas components in its polyhedral cages, which consist of hydrogen-bonded water molecules. Gas hydrate is a nonstoichiometric compound, expressed by the formula GnH2O, where G and n are gas component
and hydration number, respectively. The three main crystallographic structures of NGH are structure I (sI), structure II (sII), and structure H (sH) [Sloan, 2003]. The crystallographic structure of NGH depends on the encaged components [Davidson et al., 1986; Uchida et al., 1999; Lu et al., 2007].
[3] The cage occupancy is defined as a ratio:
(number of cages occupied by guest molecules)/ (number of total cages). Both 13C NMR and Raman techniques have been applied widely to estimate cage occupancies, not only of synthetic gas hydrates but also of NGHs [Ripmeester and Ratcliffe, 1988; Sum et al., 1997; Uchida et al., 1999; Tulk et al., 1999; Subramanian et al., 2000a, 2000b; Uchida et al., 2002; Ripmeester et al., 2005; Kim et al., 2005; Lu et al., 2005; Kida et al., 2007]. The cage occupancies are estimated from the inte-grated intensities of 13C NMR signals or Raman peaks from encaged gases using a statistical ther-modynamic model [van der Waals and Platteeuw, 1959]. Furthermore, cage occupancies enable us to give the hydration number defined as the number of water molecules per guest molecule. The assump-tion of single occupancy for all cages gives us ideal hydration numbers of 23/4 for sI, 17/3 for sII, and 17/3 for sH. The abundance of gas hydrate in sediments can be calculated from the hydration number and the density of gas hydrate under the conditions studied [Lu et al., 2005]. However, global estimations of the amount of natural gas in NGHs have been typically based on assuming fixed ideal hydration number of CH4 hydrate (n = 23/4)
[Kvenvolden, 1995]. Crystals of NGHs mainly contain CH4. They also contain smaller quantities
of heavier hydrocarbons such as C2H6, C3H8, and
i-C4H10and nonhydrocarbons such as CO2and H2S
depending on the local environment [Davidson et al., 1986]. A Knowledge of hydration numbers in various environments should improve global esti-mates of the amount of natural gas trapped in gas hydrate crystals. However, cage occupancies and hydration numbers of NGHs are poorly understood.
[4] The hydration number can be estimated by
direct measurements of the gas-water ratio obtained by dissociation of the gas hydrate. However, NGHs generally contain frozen water [Takeya et al., 2006]. Therefore, direct measurements have the potential to overestimate hydration numbers, since it is difficult to distinguish water from gas hydrate crystals from that frozen water. As an alternative to direct measurement, 13C NMR or Raman tech-niques are convenient for estimation of cage occu-pancies and hydration number without considering the excess frozen water. Since they do not measure the excess frozen water, those values are estimated from integrated intensities ratio of peaks from guest molecules interacting with host water mole-cules. Under the coexistence of three phases, gas, hydrate, and ice, the 13C NMR or Raman signals from the encaged gases are separated from signals of the gas phase [Sum et al., 1997; Subramanian et al., 2000b]. In particular, in the case of mixed gas system, the 13C NMR technique gives us high-resolution signals from mixed gas hydrates com-pared with Raman spectroscopy.
[5] For NGHs containing CH4 as a main
compo-nent, the n values of 5.91 (via direct measurement) for the Middle America Trench (off Guatemala) sample [Handa, 1988], 6.2 (via Raman) for the Blake Ridge (off South Carolina) sample [Uchida et al., 1999], 6.1 – 6.3 (via 13C NMR and Raman) for the Mallik sample [Ripmeester et al., 2005], 6.11 (via 13C NMR) for the Hydrate Ridge (off Oregon) samples [Kim et al., 2005], 6.1 (via 13C NMR and Raman) for the offshore Vancouver Island samples [Lu et al., 2005], and 5.900 (via X-ray diffraction) for the offshore Vancouver Island sam-ple [Udachin et al., 2007] have been reported. Furthermore, naturally occurring mixed gas hydrates containing heavier hydrocarbons have been discovered in the Caspian Sea [Ginsburg et al., 1992], the Gulf of Mexico [Davidson et al., 1986], the Gulf of Cadiz [Mazurenko et al., 2002], the Cascadia margin [Lu et al., 2007], and other places, but attempts to estimate the n value are reported for mixed gas hydrate recovered from the Gulf of Mexico [Handa, 1988]. The Gulf of Mexico sample contained 66% CH4, 2.9% C2H6,
14.7% C3H8, 3.7% i-C4H10, and trace amount of
heavier hydrocarbons [Davidson et al., 1986]. Handa used the gas-water ratio for the estimation and reported the n value of 8.2 for the sample. The differences in the hydration numbers between the natural mixed gas hydrate and the natural CH4
hydrates reported could be due to those in gas compositions. On the other hand, it has been
Geochemistry Geophysics Geosystems
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from the offshore Vancouver Island was attempted by X-ray diffraction technique based on the assump-tion of full cage occupancy [Udachin et al., 2007]. [6] The purpose of the present study is to obtain
occupation factors of gas hydrates formed from natural gases with different gas compositions. In this study, the NGHs recovered from different areas in Lake Baikal were studied. Lake Baikal is the only fresh water environment where NGHs have been recovered [Kuzmin et al., 1998]. In Lake Baikal, NGHs form from microbial gas and from thermogenic gas in the southern Baikal Basin (SBB) and from gas associated with deep oil fields in the central Baikal Basin (CBB) [Kuzmin et al., 1998; Kida et al., 2006]. We attempted to estimate cage occupancies and hydration numbers of pure CH4and mixed CH4-C2H6hydrates locally formed
in different areas in Lake Baikal.
2. Materials and Experimental Method
[7] Gas hydrate-bearing sediment cores were
re-covered from three study sites, the Malenky and Bolshoy mud volcanoes in the SBB and the K-2 mud volcano in the CBB of Lake Baikal (Figure 1), which were obtained by gravity coring. The field investigations were carried out in 2005 and 2006 in the framework of an international collaboration of
the Kitami Institute of Technology, Japan and the Limnological Institute SB RAS, Russia. The hy-drate fractions were stored and transported at liquid nitrogen temperature using a dry-shipper. Photo-graphs of the hydrate-bearing sediments are pre-sented in Figure 2. Massive or small piece of NGHs were found in silty clay in the cores recovered. The Malenky core was obtained at 51°55.2230N, 105°38.1090E, and water depth of 1316 m. The core had a length 124 cm and the hydrate (sample A) was obtained from the sub bottom depth interval of 111 – 124 cm. The 104-cm-long Bolshoy core was obtained at 51°52.7790N, 105°33.3370E, and water depth of 1353 m. The hydrate, which was obtained from the 87 –99 cm of the core, is denoted as sample B. The K-2 core was obtained at 52°35.4730N, 106°46.2840E, and 908.4 m water depth. The core was 295 cm long. The hydrates were obtained from three intervals: 250 –258 cm (sample C-1), 269– 273 cm (sample C-2), and 274– 279 cm (sample C-3).
[8] A portion of the hydrates stored in a
dry-shipper was decomposed in a small vessel to analyze molecular and isotopic compositions (d13C and dD) of hydrocarbons in the released gas. The experimental methods of the gas analyses were already detailed in our previous paper [Kida et al., 2006]. The confidence limits of the measure-ments of hydrocarbon composition are 0.005% for C1– C3 hydrocarbons and 0.0005% for C4– C9
hydrocarbons, which were determined by duplicate analyses of standards. The analytical precision of d13C is 0.1% and that ofdD is 0.6% [Hachikubo et al., 2007].
[9] The 13C single-pulse NMR spectra for
identi-fications of encaged hydrocarbons, determination of clathrate hydrate structure, and estimation of cage occupancies and hydration number were mea-Figure 1. Map of Lake Baikal: (a) location of Lake Baikal and (b) location and water depth of the study sites. The Malenky and Bolshoy mud volcanoes are located in the southern Baikal Basin (SBB); the K-2 mud volcano is located in the central Baikal Basin (CBB).
sured using an NMR spectrometer (100 MHz, JNM-AL400; JEOL) equipped with the probe for solid samples (SH40T6; JEOL). The hydrates were introduced into a zirconia sample tube (6 mm diameter, 22 mm length; JEOL) in liquid nitrogen. All spectra were measured at 171 K using cooled dry nitrogen gas. The values of the 13C chemical shift were determined using adamantane as an
external reference material with the methyl carbon peak at 298 K set at 29.472 ppm [Hayashi and Hayamizu, 1991]. On the basis of results of a previous study [Kida et al., 2007],13C single-pulse NMR measurements were carried out under the following conditions: 13C pulse length 5.5 ms (90°), pulse delay time 50 s for complete relaxation Figure 2. Close-up photographs of the hydrate-bearing sediment cores recovered from the three study sites. (a) Photograph of the bottom of the core recovered from the Malenky structure, displaying white massive gas hydrates. Sample A was collected from the massive gas hydrates. (b) Close-up core photograph of the interval of 80 – 108 cm below the lake floor of the core recovered from the Bolshoy structure. (c) Close-up core photograph of the interval of 246 – 297 cm below the lake floor of the core recovered from the K-2 mud volcano. Samples studied were obtained from inside of the white frame in Figures 2b and 2c.
Table 1. Molecular and Isotopic Compositions of Hydrocarbon in Dissociated Gases From Five Hydrate Samples Recovered From Lake Baikala
SBB Basin CBB Basin: K-2 Site
Malenky Site: Sample A Bolshoy Site: Sample B Sample C-1 Sample C-2 Sample C-3 Molecular Composition Methane/% 99.97 99.98 85.22 96.74 96.24 Ethane/% 0.03 0.02 14.69 3.24 3.72 Propane/% <0.01 <0.01 0.02 <0.01 <0.01 Isobutane/% ND <0.001 0.005 0.006 0.007 n-butane/% ND <0.001 0.001 0.001 0.001 Neopentane/% ND <0.001 0.064 0.001 0.001 Isopentane/% <0.001 <0.001 <0.001 0.004 0.007 Cyclopentane/% <0.001 <0.001 ND ND ND n-pentane/% ND ND <0.001 <0.001 <0.001 n-hexane/% ND ND <0.001 ND ND SC6others/% ND <0.001 ND 0.004 0.009 SC7– C9/% ND <0.001 <0.001 0.004 0.005 C1/(C2+ C3) 3593 4732 6 30 26 Isotope Composition d13C of methane/% 66.6 66.5 57.4 57.6 57.0 dD of methane/% 304 302 303 303 304 d13C of ethane/% ND ND 25.9 26.4 26.1 dD of ethane/% ND ND 216 200 200 a
ND means that the component was not detected.
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3. Results and Discussion
3.1. Composition and Isotopic Signature of the Baikal Hydrates
[10] The molecular and isotopic compositions of
hydrocarbons in dissociated gas from five hydrate samples recovered from the SBB and CBB in Lake Baikal are presented in Table 1. Also, two different relationships between isotopic ratios and composi-tional ratio of C1/(C2 + C3) in the released gas are
shown in Figure 3, based on studies by Bernard et al. [1976] and Whiticar [1999]. Samples A and B recovered from Malenky and Bolshoy in the SBB contained over 99.9% CH4. Figure 3 shows the
gases were of microbial origin by methyl-type (acetic) fermentation. The characteristics of hydro-carbons trapped in the gas hydrate samples A and B resembled those of the samples recovered during the Baikal Drilling Project (BDP) at the SBB in 1997 [Kuzmin et al., 1998]. For samples C-1, C-2, and C-3 from the CBB, high concentrations of CH4
and C2H6 were detected. The concentration ratios
CH4/C2H6 differed considerably from each of the
three samples: the ratio CH4/C2H6 of sample C-1
was 85.22/14.69, whereas those of samples C-2 and C-3 were, respectively, 96.74/3.24 and 96.24/ 3.72. The isotope values for CH4and C2H6trapped
in the three hydrate samples were almost constant, and were similar to those obtained from the same area during our study conducted in 2005 [Kida et al., 2006]. Figure 3 indicates that the gas hydrates
in the K-2 mud volcano were formed by mixing of microbial and thermogenic gas.
3.2. Crystal Structure of the Baikal Hydrates
[11] The 13C single-pulse NMR spectra of NGH
samples recovered from Lake Baikal are depicted in Figure 4. Sample A, recovered from Malenky in the SBB, showed two peaks at 4.29 ppm and
Figure 4. The 13C single-pulse NMR spectra of five hydrate samples recovered from Lake Baikal.
Figure 3. (a) Relationship between carbon isotope composition (d13C) of CH4 and the ratio C1/(C2 + C3).
6.62 ppm, which were attributed to CH4in small
and large cages of sI, respectively, based on earlier studies on synthetic gas hydrates [Ripmeester and Ratcliffe, 1988; Kida et al., 2007]. The unit cell of sI hydrate is constructed by 46 H2O molecules
(two small cages (12-hedrons) and six large cages (14-hedrons)) [Sloan, 2003]. Sample B recovered from Bolshoy in SBB yielded two peaks at 4.24 and6.62 ppm, which were assigned respectively to CH4 in small and large cages of sI as well as
sample A. For samples recovered from the K-2 mud volcano in the CBB, sample C-1 showed three peaks at 6.21 ppm, 4.33 ppm, and 8.22 ppm, which are respectively attributed to C2H6 in large
cages of sII, CH4 in small cages of sII, and CH4
in large cages of sII, similarly to the spectra of CH4-C2H6 mixed gas hydrates already reported
[Subramanian et al., 2000a, 2000b; Kida et al., 2006, 2007]. The unit cell of sII hydrate consists of 136 H2O molecules (16 small cages (12-hedrons)
and eight large cages (16-hedrons)) [Sloan, 2003]. The C-2 sample yielded three peaks at different positions from those of sample C-1. The three peaks of sample C-2 at chemical shifts of 7.91 ppm, 3.99 ppm, and 6.47 ppm, were attributed respec-tively to C2H6 in large cages of sI, CH4 in small
cages of sI, and CH4 in large cages of sI. The
spectrum of sample C-3 showed three peaks at the same positions as those of sample C-2, indicating the sI hydrate crystal structure. These 13C NMR spectra indicated that sI and sII hydrates coexist in the core recovered from the K-2 mud volcano. This observation was similar to that in our previous report on hydrate-bearing cores recovered from the K-2 mud volcano [Kida et al., 2006].
3.3. Cage Occupancies and Hydration Numbers of the Baikal Hydrates
[12] In the CH4-C2H6 system, the values of small
cage occupancy of CH4 (qM,S), large cage
occu-pancy of CH4 (qM,L), and large cage occupancy of
C2H6 (qE,L) are given by relative occupancy ratios
calculated by13C NMR signals from encaged CH4
and C2H6, based on a statistical thermodynamic
model [van der Waals and Platteeuw, 1959]. Details of this estimation have been described in the paper reported by Kida et al. [2007]. This method have several assumptions: free energy of the hydrate is independent of cage occupation, distortions of hydrate cages and guest-guest inter-actions are excluded, and each cage can contain one guest molecule [Sloan, 1998]. The hydrate formation temperature in this study was assumed to be the reported water temperature (276.6 K) of the bottom of Lake Baikal [van Rensbergen et al., 2002]. The hydration number, n, is given by equation (1) for sI and equation (2) for sII.
n¼ 23 3 qM ;Lþ qE;L þ qM ;S ð1Þ n¼ 17 qM ;Lþ qE;L þ 2qM ;S ð2Þ
Here, the values of qE,L for samples A and B
recovered from the SBB are not included because no signals from C2H6 were observed.
[13] The cage occupancies and hydration numbers
of NGHs recovered from Lake Baikal are presented in Table 2. The cage occupancies of sample A (sI)
Table 2. Summary of Cage Occupancies and Hydration Numbers of Five Hydrate Samples Recovered From Lake Baikala
SBB Basin CBB Basin: K-2 Site Malenky Site:
Sample A
Bolshoy Site:
Sample B Sample C-1 Sample C-2 Sample C-3
Crystal structure I I II I I
CH4/C2H6composition
in dissociated gas (%)
99.97/0.03 99.98/0.02 85.22/14.69 96.74/3.24 96.24/3.72 Small cage occupancy
of CH4,qM,S
0.853 ± 0.030 0.818 ± 0.025 0.732 ± 0.031 0.775 ± 0.032 0.798 ± 0.080 Large cage occupancy
of CH4,qM,L
0.975 ± 0.002 0.977 ± 0.001 0.574 ± 0.029 0.943 ± 0.013 0.916 ± 0.058 Large cage occupancy
of C2H6,qE,L
– – 0.412 ± 0.026 0.036 ± 0.012 0.035 ± 0.004 Hydration number, n 6.09 ± 0.04 6.14 ± 0.04 6.94 ± 0.17 6.20 ± 0.05 6.17 ± 0.11
a
The uncertainties are the standard deviations of duplicate determinations.
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kida et al.: lake baikal hydrates with different properties10.1029/2009GC002473qM,S = 0.853 andqM,L= 0.975. The value of n for
sample A was 6.09, which resembles values reported for synthetic pure CH4hydrate (n = 6.05 ±
0.06 [Ripmeester and Ratcliffe, 1988]) and NGHs containing CH4 as a main component (n values of
6.11 for the Hydrate Ridge (offshore Oregon) sample [Kim et al., 2005] and 6.1 for the offshore Vancouver Island samples [Lu et al., 2005]). For sample B (sI) recovered from the SBB, the values ofqM,S,qM,L, and
n were 0.818, 0.977, and 6.14, respectively, which were almost consistent with those of sample A. The present results for samples A and B showed nearly full CH4occupation for large cages and the presence
of some vacant small cages in sI hydrate. For sample C-1 (sII), recovered from the CBB, the cage occu-pancies were estimated asqM,S= 0.732,qM,L= 0.574,
and qE,L = 0.412. The sum of qM,L and qE,L was
0.986, reflecting that CH4and C2H6almost entirely
occupied the large cages in sII. The value of n for sample C-1 was 6.94, which is much greater than those for samples A and B with sI. Nevertheless, compared to the reported n value of 8.2 for the sII naturally occurring mixed gas hydrate recovered from the Gulf of Mexico, the value of n for sample C-1 (sII) was quite small. This difference in n values between sample C-1 and the Gulf of Mexico sample is inferred to result from the differences in gas composition. Furthermore, our value, which was estimated without considering the effect of excess water in the sample, is expected to differ greatly from the value estimated by Handa using the gas-water ratio. For sample C-2 (sI) recovered from the CBB, the values of qM,S, qM,L, and qE,L were,
respectively, 0.775, 0.943, and 0.036. The sum of qM,L and qE,L was 0.979, indicating that the large
cages of sI were almost at full occupancy level. The hydration number of sample C-2 was estimated as n = 6.20, which is slightly greater than those of samples A and B. The cage occupancies and hydration number of sample C-3 (sI) located directly below sample C-2 were almost identical to those of sample C-2 (sI), as listed in Table 2. The difference in the values ofqM,Samong the CH4-C2H6hydrates
from the K-2 mud volcano are not large, but, in contrast, the differences in hydration number between them is quite large. This difference is due to the fact that number of small cages per a unit cell of sII is larger than that of sI.
4. Conclusions
[14] We reported on the qualitative and quantitative
parameters of the hydrocarbon gases trapped in the
amounts of both CH4 and C2H6,were pointed out
in the Baikal mud volcano hydrates, with pure CH4
hydrates in southern Baikal Basin and mixed CH4
-C2H6 hydrates in central Baikal Basin. Their cage
occupancies and hydration numbers were estimated using an optimal 13C NMR method. All samples studied had small cage occupancy of CH4ranging
from 0.732 to 0.853. Large cages were almost fully occupied by only CH4or two guest components of
CH4 and C2H6 for all samples. The difference in
the values of qM,S among the NGHs studied
resulted in large differences among the hydration numbers. The present study suggests that consid-eration of cage occupancies and hydration numbers of mixed gas hydrates in local environments can be used to improve global estimates of the amount of natural gas trapped in hydrate crystals.
Acknowledgments
[15] This work was supported by funding agencies in Japan (Japan Society for the Promotion of Science KAKENHI 17550069, 18206099, 19550077, and 19740323 and the Kitami Institute of Technology Presidential Grants), in Russia (Integration Project of Siberian Branch of Russian Academy of Sciences 58), and in Belgium (Flemish Fund for Scientific Research (FWO-Vlaanderen) and the Bilateral Flemish – Russian Federation Project). One of the authors (M. K.) thanks Jiro Nagao (AIST) for fruitful discussions.
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